While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. The organic layers in earlier devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material—dopant, which results in an efficiency improvement and allows color tuning.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as operational lifetime, color, luminance efficiency and manufacturability.
Notwithstanding these developments, there are continuing needs for organic EL device components that will provide better performance and, particularly, long operational lifetimes. This is especially true for phosphorescent emitter-containing LEL, particularly blue phosphorescent emitter-containing LEL. There are a number of approaches to achieve better operational lifetimes disclosed in prior publications. An improvement in operational stability due to admixing hole transport material to emissive electron transport was reported by Z. Popovic et al. in Proceeding of the SPIE, vol. 3476, 1998, p. 68-73. An improvement in both device efficiency and operational lifetime was reported to result from doping emissive layer by fluorescent dye such as dimethylquinacridone [J. Shi and C. W. Tang Appl. Phys. Lett., vol. 70, 1997, p. 1665-1667]. Further improvements in operational lifetime of the devices doped with fluorescent dyes were realized by co-doping emissive layer with anthracene derivatives [JP 99273861, JP 284050]. Co-doping by rubrene has been reported to result in 60% increase in operational half-life of the device doped with red fluorescent dye DCJTB [EP 1162674]. This improvement is still insufficient for many commercial applications of the OLED devices. It is desirable to achieve further improvements in OLED stability.
The conversion of electrical energy into light is mediated by excitons. An exciton is like a two particle system: one is an electron excited into an unfilled higher energy orbital of a molecule while the second is a hole created in the ground state due to the excitation of the electron. However, excitons also play an important role in the failure of high efficiency OLED devices. Thus exciton management is essential for improved OLED performance. In their 2012 review, S. Reineke, and M. A. Baldo (S. Reineke and M. A. Baldo, Phys. Status Solidi A 209, 12. 2012 2341-2353) identified three processes that account for triplet exciton quenching: (1) Triplet-polaron quenching (TPA); (2) Electric field induced exciton dissociation; (3) Triplet-triplet annihilation (TTA).
Of the three processes, TTA is the only process that scales with the square of the exciton density, and dominates the decrease in efficiency at high exciton densities (efficiency roll-off). TTA in a doped film can have different underlying mechanisms. One of them is a single-step long-range interaction (dipole-dipole coupling), based on Förster-type energy transfer. The rate of TTA energy transfer is proportional to the spectral overlap of the phosphorescent emission of the donor and the absorption of the acceptor excited triplet state. In a host-guest system where the triplet level of the host is higher than the guest, the single-step long-range mechanism should be the only channel of TTA for typical guest concentrations ranging from 1 to 10 mole %.
An additional TTA channel is mediated by hopping-assisted migration (Dexter-type energy transfer) of triplet excitons in clusters of guest molecules. This mechanism was identified in a TCTA:Ir(ppy)3 guest-host system at concentration around 10 mole %. High angle annular dark field (HAAD) TEM has indeed revealed the presence of Ir(ppy) clusters and aggregation.
More recently, Y. Zhang, and H. Aziz (Yingjie Zhang, and Hany Aziz, ACS Appl. Mater. Interfaces, 2016) reported that the degradation mechanisms in blue PHOLEDs are fundamentally the same as those in green PHOLEDs. Their investigations show that quantum yield of both the host and the emitter in the EML degrade due to exciton-polaron interactions, and that the deterioration in material quantum yield plays the primary role in device degradation under operation. The results show that charge balance is also affected by exciton-polaron interactions, but the phenomenon plays a secondary role in comparison. They concluded that the limited stability of the blue devices is a result of faster deterioration in the quantum yield of the emitter.